Same problem here, this api flowable-rest/service/form/form-data can’t access to form data, I use flowable-task/form-api/form/form-instance-model instead, it’s documented in https://github.com/flowable/flowable-engine/blob/eeebfd23af3571741b60e3f663f17c0ecc50a812/docs/public-api/references/swagger/form/flowable-swagger-form.yaml#L439-L462
I’m using https://www.flowable.org/docs/userguide/index.html#_submit_task_form_data to save a task form.